Fet-Based Gas Sensor

ABSTRACT

The invention relates to an FET-based gas sensor comprising a gas channel for diffusing a measuring gas to a gas-sensitive layer which is actively connected to a FET for signal readout. According to the invention, an electrochemical element is at least partially inserted into the gas channel for the electrochemical conversion of interfering gases. The arrangement is permeable to the target gas.

Gas sensors based on field effect transistors measure the change in work function between a gas-sensitive material and a reference material. FIG. 6 shows a typical structure. The gateless transistor is coupled capacitively, through an air gap constituting the gas inlet a few μm wide, to a gas-sensitive layer. Upon contact of the gas with this sensitive layer, the surface potential changes, whereby the transistor is controlled. The working temperatures of such sensors are between ambient temperature and 150° C.

FIG. 7 shows an embodiment of such a gas sensor structure as a so-called SGFET (suspended gate FET), which is disclosed, for example, in DE 42 39 319 C2.

Other structures use a structure in which the air gap with the gas-sensitive layer and the FET structure for readout are separated, as shown in FIG. 8, for example. Such structures, called CCFET structures, are disclosed by DE 4333875 C2.

The structure of a gas sensor based on the field effect transistor is shown schematically according to FIG. 8 using the example of a CCFET—prior art. The readout FET, illustrated by the source S and drain D contacts, is controlled by an uncontacted gate that together with the opposite gas-sensitive electrode constitutes a capacitor system.

In all of these gas sensors, in addition to reactions to target gases to be measured, undesired signals from other gases also appear frequently, which under some circumstances are superimposed on the useful signal in multiple intensity and thus lead to incorrect measurements. Furthermore, the signal level to the target gas may be modified by the presence of an interfering gas.

Improvements in the prior art for avoiding drawbacks have consisted of:

-   -   optimization of the sensitive materials.

By optimizing the gas-sensitive materials and the reference materials located on the transistor, the influence of interfering gases can be reduced to some extent with regard to selectivity.

-   -   compensation by means of a reference sensor.

If the interfering gas is known, the influence of the interfering gas can be offset by a second sensor sensitive to this gas. In reality, this second sensor again has only limited sensitivity, so that additional cross sensitivities are imparted to the sensor system. Also, the reference coating has little promise when the level of the interfering signal exceeds the useful signal by a multiple.

-   -   the use of filters.

Interfering gases may be permeable to the target gas but filters for interfering gases may suppress them. In this case, on the one hand, the interfering gas is removed by activated charcoal, for example. However, the limited filter capacity here has to be considered in long-term operation, and the gas can break through the filter when the filter capacity is reached. In the case of sensors that are operated at elevated temperature, the interfering gas may be converted by a chemical reaction to components that cause no sensor signal, for example such as catalytic filters to decompose alcohols as disclosed by DE 43 10 914. Such filters do not have the aforementioned problems when filter capacity is reached. To be able to utilize such filters, however, temperatures >300° C. are needed, for one thing, so that such methods cannot be used for structural elements with Si chips and a maximum operating temperature of about 150° C.

The goal of this invention is to increase the selectivity of FET-based gas sensors.

This is accomplished both by the combination of features of Claim 1, and by those of Claim 11. Advantageous refinements are described in the subclaims.

In general, the invention supplements the structure of an FET-based gas sensor, of FETs according to the prior art according to FIG. 7 or FIG. 8, by improvement by means of a channel through which the gas has to diffuse in order to reach the detection area of the sensor. There is a sensitive layer in this channel/air gap, and at the same time an electrochemical element is introduced into this channel that decomposes interfering gases by selective electrochemical conversion and allows the target gas to pass through at the same time.

Actually, the gas channel for feeding the measurement gas is made by lengthening the air gap between the channel insulation of the FET and the sensitive layer on a carrier substrate applied by hybrid technology. The electrochemical element introduced in the gas feed channel to the sensitive area consists of at least two active electrodes to which a voltage is applied, at least one of which must be in direct contact with the relevant measured gas mixture, and at least one of which is contacted with an active ion conductor at a typical operating temperature of the FET.

The limitation of the “inward-directed” gas diffusion is great enough so that the electrochemical element is able adequately to convert interfering gas in the application to a gas no longer detectable in the active sensor area.

When a proton conductor is used (hydrogen ion conductor), both oxidizable and reducible interfering gases can be degraded in a comparable manner.

Exemplary embodiments will be described below with reference to the accompanying Figures that do not limit the invention.

FIG. 1 shows schematically the basic structure of one sensor according to the invention with an ion conductor and active electrodes El 1 and El 2,

FIG. 2 shows an implementation of the electrochemical filter with one electrode in the gas diffusion channel and a second electrode outside the gas diffusion channel,

FIG. 3 shows an implementation of the electrochemical filter with a pair of electrodes in the gas diffusion channel on the ceramic carrier substrate,

FIG. 4 shows an implementation of the electrochemical filter with separate electrodes on the ceramic carrier substrate and on the sensor chip,

FIG. 5 shows a possible plan view for the variant proposed in FIG. 4 with annular active electrodes,

FIG. 6 shows schematically the structure of a gas sensor based on the field effect transistor with the example of an SGFET (suspended gate FET) according to the prior art,

FIG. 7 shows the known embodiment of an SGFET,

FIG. 8 shows schematically the structure of a gas sensor based on a field effect transistor with the example of a CCFET according to the prior art.

FIG. 1 illustrates the basic structure of an FET according to the invention. This schematic basic structure of a gas sensor in the figure provides for an electrochemical element with the ion conductor and active electrodes El 1 and El 2, on the right next to the detection area with sensitive layer and transducer (e.g. CCFET or SGFET).

In the structure shown in FIG. 1, the electrode El 2 of the electrochemical structure is located in the air gap and selectively converts interfering gas, while the electrode El 1 is located outside the air gap and provides for charge compensation in the ion conductor by a complementary reaction.

Materials for the Structure Used:

The carrier substrate here is usually any desired nonconductor that must only allow the preparation of the layers placed on it. Examples of this are ceramics, for example such as Al₂O₃, AlN, or Si₃N₄, glass, or suitable polymeric materials, for example conventional PCB materials like FR4 or the like.

The active electrodes, at least on their surface, consist of a stable and catalytically active metal. Metals that are well suited in general are metals of the platinum group and their alloys, for example Pt, Pd, Pt/Rh, or silver.

The layer joining the two electrodes consists of a material that conducts ions in the temperature range below 150° C.

The entry of gas and its diffusion to the sensor area is limited so sharply by the geometry of the gas inlet that the small amounts of gas can be reacted by the electrochemical filter and be carried forward, and an amount of target gas that is sufficient for its detection is allowed to pass through.

An operating procedure for the selective detection of a target gas in a gas mixture will be described, in which the target gas is detected in the sensor area that is bounded by the gas-sensitive layer and the readout transistor, and in which the measured gas mixture to be examined is previously so modified by the electrochemical filter described above that selective detection of the target gas in the sensor area is possible.

Basic Function of the Electrochemical Filter:

Depending on the polarity of the applied d.c. voltage U_(diss), one of the two electrodes El 1 or El 2 acquires an oxidizing action on the gas diffusing in the gap, and the other acquires a reducing action. For example, if El 2, which is in the gap according to FIG. 1, is chosen as the oxidizing electrode, interfering gases that are more readily oxidizable than the target gas can thereby by converted by oxidation into gases that no longer cause cross sensitivity. It is important here that both the nature of the behavior of the electrodes and also their oxidizing or reducing powers can be determined by the choice of the voltage U_(diss).

Of course only oxidizable gases that are more easily oxidizable than the target gas can be removed by the procedure described. The same applies similarly to reducible gases.

Examples of common gases that can be removed by electrochemical reaction are shown in the following list: Original gas Converted to Reduction of the gas by electrode with reducing action in the gas diffusion gap NO₂ NO NO N₂ O₃ O₂ Oxidation of the gas by electrode with oxidizing action in the gas diffusion gap HC (Hydrocarbons) H₂O and CO₂ Alcohols, ketones, aldehydes H₂O and CO₂ H₂ H₂O NH₃ N₂ and H₂O CO CO₂

The electrochemical reaction of various gas components can be brought about by an electrochemical filter by selective application of a voltage of given polarity and magnitude.

Depending on the polarity of the electrodes, this system can be used either for the degradative reaction of oxidizing gases or for the degradative reaction of reducing gases. The magnitude of the voltage to be applied in this case is typically between 200 mV and 2 V.

When using an oxygen ion conductor, there is often the desire to detect gases with reducing action or CO₂ with FET gas sensors. In this case, the NO₂ with oxidizing action that often occurs at the same time in the application may lead to distinct interfering reactions in many FET gas sensors. The reactive NO₂ can be converted to the usually non-interfering NO according to the equation NO₂+2e⁻→NO+O²⁻ by applying a negative voltage to El 2. The NO can also be decomposed with no residue by a stronger negative voltage: 2NO+4e⁻→N₂+2O²⁻

The oxygen is taken up by the ion conductor.

Gaseous oxygen is formed on the electrode El 1 as a counterreaction. 2O²⁻−4e⁻→O₂

With reversed polarity, unwanted reducible gases can be removed on El 1, for example according to the following equations 2NH₃+3O²⁻→N₂+3H₂O+6e⁻ 2H₂+2O²⁻→2H₂O+4e⁻

As a counterreaction on the electrode El 1, the gaseous oxygen is taken up. O₂+4e⁻→2O²⁻

By Using a Proton Conductor (Hydrogen Ion Conductor):

-   -   Both oxidizable and reducible interfering gases can be degraded         in a comparable manner.     -   NO₂ is degraded in this case on El 2 in the presence of hydrogen         or of another gas containing hydrogen such as NH₃, CH_(x), etc.,         for example by         NO₂+2e⁻+2H⁺→NO+H₂O.

As a counterreaction, gaseous hydrogen is taken up as hydrogen ion in the proton conductor at the electrode El 1. H₂→2H⁺+2e⁻.

The above reaction can of course be used directly to remove hydrogen if the electrodes wired in this way are in the gap. Similarly, NH₃ or hydrocarbon HC, for example, can be oxidized with an oxidizing connection of El 1. 2NH₃→N₂+6H⁺+6e⁻ 6H⁺+6e⁻→3H₂

The comparable mechanisms also apply to other ion conductors.

Materials that have ion-conducting properties in the temperature range below 150° C. are preferred as materials for the ion conductor. Examples are listed below:

An example of an oxygen ion conductor is LaF₃.

Hydrogen Ion Conductors:

For example, hydrogen uranyl phosphate tetrahydrate (HUO₂PO₄.4H₂O), NH₄TaWO₆, NAFION, aluminum silicate+Na₂O or Li₂O or K₂O, ion-conducting sodium compounds such as Nasicon Na_(1+x)Zr₂P_(3−x)Si_(x)O₁₂,Na₅YSi₄O₁₂;

ion-conducting lithium compounds such as lithium nitride, lithium titanium phosphate.

Special Advantages of the Invention:

The selectivity of a gas sensor system can be substantially increased by the system described by removing gases that lead to incorrect measurements.

Electrochemical filters have the conspicuous advantage over ordinary filters that they do not become consumed or saturated, and that more stable long-term continuous operation is therefore possible. In addition, they are smaller in structural shape.

Electrochemical filters are not statically acting filters; their simple control by the dissociation voltage U_(diss) permits the implementation of self-monitoring and self-calibrating functions.

With the gas sensors according to the invention, there is the desire to keep the infeed gas diffusion as small as possible to bring about complete removal of interfering gases with the least possible electrochemical filter power. This is accomplished by the form of embodiment according to FIG. 2. The ion conductor here fills up the entire gas diffusion gap, so that it is possible to provide for gas access to the sensitive layer with the prevailing open-pore structure.

FIG. 2 shows the embodiment of the electrochemical filter with one electrode in the gas diffusion channel and a second electrode outside of the diffusion channel on the ceramic carrier substrate. Gas access is through the porous ion conductor.

FIG. 3 illustrates an interesting refinement of the invention. In this arrangement, both electrodes are positioned in the narrow gas diffusion gap. With an electrochemical filter with this configuration, both reducing and oxidizing gases are decomposed. The decomposition thresholds can be set by means of the applied potential. Such an arrangement is particularly suitable for detecting inert gases like CO₂, for example, that are unchanged by the filter, selectively with respect to their redox behavior.

FIG. 4 illustrates an embodiment in which the characteristics of FIG. 2 and FIG. 3 are combined. Both electrodes are located in the air gap/measured gas channel, while gas access is through the porous ion conductor. Furthermore, in this case the geometry of the electrodes is modified in that the oppositely placed electrodes have a larger surface area and the interaction between gas and electrode is thereby improved.

FIG. 5 shows a possible embodiment of the electrodes as annular structures. This makes for a substantially larger gas inlet than a structure closed on one side, and thus the gas exchange is improved to the benefit of the response time.

Multistage electrochemical filters are also provided for in the structures according to the invention; in other words there are multiple El 2s in the gas diffusion gap that consist of different materials and/or are operated at different voltages.

Self-monitoring and Self-calibration Functions:

The filtering action of the electrochemical filters is produced selectively by the electrical control of El 1 and El 2. The filtering action can be varied by varying the control voltage U_(diss), which itself can be used for the reaction to remove the target gas.

In addition to self-monitoring functions, this also makes sequential measurement of multiple gases possible, for example by continuously increasing the filtering action.

If the electrochemical filters for the gas to be detected are so designed, the target gas can be removed from the detection area by modulation of the control voltage, and an artificial zero point can be provided in this way. Baseline variations of the sensor can thus be eliminated efficiently.

Activation of the Target Gas

An undetectable gas can be converted into a gas to which the gas-sensitive material reacts by the system described. 

1. FET-based gas sensor with a gas channel for the diffusion of a gas to be measured to a gas-sensitive layer that is in active connection with an FET for readout of the signal, in which an electrochemical element is at least partially introduced within the gas channel for the electrochemical conversion of interfering gases and/or to activate a target gas, with the system being permeable to the target gas.
 2. Gas sensor according to claim 1 in which the channel for the gas to be measured is made by lengthening the air gap between the channel insulation of an FET and the suspended gate electrode with the gas-sensitive layer.
 3. Gas sensor according to one of the foregoing claims in which the electrochemical element is positioned in the cross section of the gas measurement channel in such a way that it does not completely fill it at any point.
 4. Gas sensor according to one of the foregoing claims in which the electrochemical element completely occupies the cross section of the gas measurement channel and consists of porous material.
 5. Gas sensor according to one of the foregoing claims in which the electrochemical element is made up of an ion conductor and at least two active electrodes to which an external voltage is applied, wherein the two electrodes separated from one another are connected to the ion conductor and at least one of the electrodes is in direct contact with the gas to be measured.
 6. Gas sensor according to claim 5 in which one electrode is positioned in the gas measurement channel to convert interfering gas, and the other electrode is positioned outside the gas measurement channel for a complementary reaction for charge compensation in the ion conductor.
 7. Gas sensor according to claim 5 or 6 in which the ion conductor consists of a material that is stable to temperatures up to 150° C.
 8. Gas sensor according to one of claims 5-7 in which the electrodes consist of a mechanically stable and catalytically active metal, at least on the surface.
 9. Gas sensor according to one of claims 5-8 in which the ion conductor is a proton conductor.
 10. Gas sensor according to one of the foregoing claims in which the electrochemical elements are of multistage design.
 11. Operating method for the selective detection of a target gas in a gas mixture to be measured by means of a field effect transistor with a gas-sensitive layer integral with a suspended gate, wherein the measurement gas mixture to be examined is previously prepared by an electrochemical element in such a way that the gas to be measured contains minimal amounts of gases that cause interfering gas reactions besides the target gas, and/or at least one target gas is activated so that it can be detected by a gas-sensitive material.
 12. Operating method according to claim 11 in which an external voltage is applied to the electrodes and one of the electrodes has oxidizing action and the other has reducing action, wherein the electrode chosen from them for the elimination of corresponding interfering gases has to be positioned in the gas measurement channel.
 13. Operating method according to claim 11 or 12 in which the potential of the external voltage is varied to match the dissociation or decomposition thresholds for given gases.
 14. Operating method according to one of claims 11-13 in which the filtering action of the electrochemical filter is selectively controlled by the electrical control of El 1 and El 2 for the self-monitoring or self-calibration of the sensor, wherein the target gas is electrochemically converted in extreme cases. 